I have no idea if you mean cinnamon added to already brewed coffee or cinnamon added before the brewing process. But just in case you don't know this trick, when you brew regular drip coffee, adding about a tablespoon of ground cinnamon to the grounds in the filter basket gives it a VERY mild cinnamon flavor, but more importantly combats some of the bitterness of automatic drip brewing and gives the whole pot a LOVELY mellow flavor.

If you meant sprinkling cinnamon on coffee afterward - yeah, you gotta be careful with that. :) Just a sprinkle will do ya.